Methylglyoxal: A Relevant Marker of Disease Activity in Patients with Rheumatoid Arthritis

Women suffering from systemic lupus erythematosus are characterized by low blood levels of α-dicarbonyl compounds

Introduction

Systemic lupus erythematosus (SLE) is a chronic, autoimmune disease, often characterised by severe course and unclear etiopathogenesis. The reaction of protein glycoxidation, also known as glycation, may be linked to etiopathogenesis of SLE. Advanced glycation end-products (AGEs) exhibit cytotoxic properties, affect cellular signalling, impair functions of extracellular proteins, and may act as neoepitopes. Glucosone (GS), glyoxal (GO), and methylglyoxal (MGO) are examples of α-dicarbonyl compounds (α-DCs) partaking in glycoxidation. The study aimed to evaluate concentrations of these three compounds in blood serum of SLE patients, and to compare the results with healthy individuals.

Material and methods

31 women suffering from SLE and 26 healthy individuals were included in the study. High-performance liquid chromatography with fluorescence detection was applied to evaluate concentrations of α-DCs in their serum samples. Correlations between the results and parameters such as disease duration time, age, glomerular filtration rate (GFR), Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K), and creatinine were analysed.

Results

The SLE patients exhibited lower concentrations of glucosone, glyoxal, and methylglyoxal than the control group. Analysis of correlations showed a difference between the examined groups.

Conclusions

In women suffering from SLE the course of α-DCs metabolism is altered. SLE patients are characterised by low serum levels of α-DCs. We hypothesise that either hindered proteasomal degradation or fast consumption of α-DCs in oxidative conditions may cause the observed low concentration of these compounds.

Albumin from sera of rheumatoid arthritis patients share multiple biochemical, biophysical and immunological properties with in vitro generated glyco-nitro-oxidized-albumin

The purpose of the present study is to explore the effects of endogenous stressors on structure and function of rheumatoid arthritis (RA) patients' albumin. In contrast to glycated-albumin or nitro-oxidized-albumin, high titre antibodies against glyco-nitro-oxidized-albumin were found in the sera of RA patients. Also, compared to the other two modified forms of albumin, glyco-nitro-oxidized-albumin showed highest percent inhibition. Albumin isolated from RA patients' sera displayed hyperchromicity and quenching of tyrosine and tryptophan fluorescence. Fluorescence spectroscopy studies also revealed the presence of dityrosine and advanced glycation end products in RA patient's albumin. RA patients' albumin showed weaker binding with 1-anilinonaphthalene-8-sulfonic acid dye. Secondary structure alterations were demonstrated by circular dichroism and Fourier transform infrared spectroscopy. Biochemical investigations revealed substantial decline in the availability of free side chains of amino acid residues; increased carbonyls and decreased sulfhydryls in RA patients' albumin. The functional impairment in RA patients' albumin was revealed by their low binding with bilirubin and cobalt. Liquid chromatography mass spectrometry analysis revealed the presence of Nε-(carboxymethyl) lysine and 3-nitrotyrosine in RA patients' albumin. The amyloidogenic aggregation of RA patients' albumin was confirmed by Congo red absorption and thioflavin-T fluorescence assays. The morphology of the aggregates was visualized under scanning and transmission electron microscope. From the above findings, we inferred that endogenous stress in RA patients have modified albumin and produce structural/functional abnormalities. Also, the presence of anti-glyco-nitro-oxidized-albumin antibodies along with other clinical features may be used as biomarker for the diagnosis and assessment of treatment responses in RA patients.Communicated by Ramaswamy H. Sarma.

Methylglyoxal suppresses microglia inflammatory response through NRF2-IκBζ pathway

Methylglyoxal (MGO) is a highly reactive metabolite generated by glycolysis. Although abnormal accumulation of MGO has been reported in several autoimmune diseases such as multiple sclerosis and rheumatoid arthritis, the role of MGO in autoimmune diseases has not yet been fully investigated. In this study, we found that the intracellular MGO levels increased in activated immune cells, such as microglia and lymphocytes. Treatment with MGO inhibited inflammatory cell accumulation in the spinal cord and ameliorated the clinical symptoms in EAE mice. Further analysis indicated that MGO suppressed M1-polarization of microglia cells and diminished their inflammatory cytokine production. MGO also inhibited the ability of microglial cells to recruit and activate lymphocytes by decreasing chemokine secretion and expression of co-stimulatory molecules. Furthermore, MGO negatively regulated glycolysis by suppressing glucose transporter 1 expression. Mechanically, we found that MGO could activate nuclear factor erythroid 2-related factor 2 (NRF2) pathway and NRF2 could bind to the promoter of IκBζ gene and suppressed its transcription and subsequently pro-inflammatory cytokine production. In conclusion, our results showed that MGO acts as an immunosuppressive metabolite by activating the NRF2-IκBζ.

The Potential Influence of Advanced Glycation End Products and (s)RAGE in Rheumatic Diseases

Advanced glycation end products (AGEs) are a class of compounds formed by nonenzymatic interactions between reducing sugars and proteins, lipids, or nucleic acids. AGEs can alter the protein structure and activate one of their receptors, specifically the receptor for advanced glycation end products (RAGE). These phenomena impair the functions of cells, extracellular matrix, and tissues. RAGE is expressed by a variety of cells and has been linked to chronic inflammatory autoimmune disorders such as rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome. The soluble (s)RAGE cleavage product is a positively charged 48-kDa cleavage product that retains the ligand binding site but loses the transmembrane and signaling domains. By acting as a decoy, this soluble receptor inhibits the pro-inflammatory processes mediated by RAGE and its ligands. In the present review, we will give an overview of the role of AGEs, sRAGE, and RAGE polymorphisms in several rheumatic diseases. AGE overproduction may play a role in the pathogenesis and is linked to accelerated atherosclerosis. Low serum sRAGE concentrations are linked to an increased cardiovascular risk profile and a poor prognosis. Some RAGE polymorphisms may be associated with increased disease susceptibility. Finally, sRAGE levels can be used to track disease progression.

Studies on the synergistic action of methylglyoxal and peroxynitrite on structure and function of human serum albumin

Albumin, an important serum protein, is continuously exposed to various oxidizing/nitrating and glycating agents. Depending upon the nature/concentration of reactive species present, the protein may be glycated, oxidized/nitroxidized or glyco-nitro-oxidized. Peroxynitrite is a powerful nitroxidant and has been reported to damage a wide array of macromolecules. On the other hand, methylglyoxal is a very strong reactive dicarbonyl and a potent precursor for the formation of advanced glycation end products under pathological conditions. In certain pathological conditions albumin may be modified by peroxynitrite and methylglyoxal simultaneously. There is dearth of literature suggests that structural/conformational and functional alteration in albumin upon glycation and oxidation/nitroxidation, however the alterations produced by glyco-nitro-oxidation has not yet been explored. Therefore, in this study, simultaneous effect of glycation and nitroxidation on the structure and conformation, vis-a-vis function of albumin was explored. Glyco-nitro-oxidized albumin showed decreased free amino acid content together with decreased affinity of albumin towards cobalt. Molecular docking model and molecular dynamic simulations showed close interaction and formation of stable complexes between methylglyoxal, peroxynitrite and albumin. Formation of carboxymethyl lysine and 3-nitrotyrosine in glyco-nitro-oxidized albumin were confirmed by MALDI-TOF MS and UP-LC MS. Aggregate formation in glyco-nitro-oxidized albumin was visualized by transmission electron microscopy. On the basis of these results, it may be speculated that, albumin modified with endogenously generated methylglyoxal and peroxynitrite might be a driving factor in the progression of heightened inflammatory autoimmune responses. The work presents a ground to study the role of glyco-nitro-oxidized albumin in the pathogenesis and progression of various autoimmune diseases including rheumatoid arthritis. Communicated by Ramaswamy H. Sarma.

Hazardous Chemical Compounds in Cookies: The Role of Sugars and the Kinetics of Their Formation during Baking

Baking goods are an essential part of the diet worldwide and are consumed daily, so they represent ideal foods for vehicle health- and unhealth-promoting substances. This work aimed to study the influence of sugars and baking conditions of cookies on the final levels of the main reported hazardous chemical compounds such as 5-hydroxymethylfurfural (HMF), 3-deoxyglucosone (3-DG), glyoxal (GO) and methylglyoxal (MGO). The replacement of sucrose with fructose or glucose in the cookies recipe deeply modifies the levels of α-dicarbonyl compounds (DCs), particularly 3-DG, independently of the baking temperature used. A longer baking time, even a few minutes, can drastically modify the HMF level in cookies and the use of fructose or glucose in the recipe seems to ensure the optimal conditions for generating this compound. The use of sucrose is required to keep levels of the hazardous compounds below a few mg/kg. Additionally, the ability to retain water, the titratable acidity and/or the pH of the final products were influenced by the used sugars with effects on the final levels of DCs and HMF. The highest Ea values determined for DCs and HMF formation in the cookies with sucrose suggest that this system requires very high temperatures to increase meaningful levels of these molecules, limiting their accumulation.

Methylglyoxal and Its Adducts: Induction, Repair, and Association with Disease

Metabolism is an essential part of life that provides energy for cell growth. During metabolic flux, reactive electrophiles are produced that covalently modify macromolecules, leading to detrimental cellular effects. Methylglyoxal (MG) is an abundant electrophile formed from lipid, protein, and glucose metabolism at intracellular levels of 1-4 μM. MG covalently modifies DNA, RNA, and protein, forming advanced glycation end products (MG-AGEs). MG and MG-AGEs are associated with the onset and progression of many pathologies including diabetes, cancer, and liver and kidney disease. Regulating MG and MG-AGEs is a potential strategy to prevent disease, and they may also have utility as biomarkers to predict disease risk, onset, and progression. Here, we review recent advances and knowledge surrounding MG, including its production and elimination, mechanisms of MG-AGEs formation, the physiological impact of MG and MG-AGEs in disease onset and progression, and the latter in the context of its receptor RAGE. We also discuss methods for measuring MG and MG-AGEs and their clinical application as prognostic biomarkers to allow for early detection and intervention prior to disease onset. Finally, we consider relevant clinical applications and current therapeutic strategies aimed at targeting MG, MG-AGEs, and RAGE to ultimately improve patient outcomes.

Modulation of 1,2-Dicarbonyl Compounds in Postprandial Responses Mediated by Food Bioactive Components and Mediterranean Diet

Glycoxidative stress with the consequent generation of advanced glycation end products has been implied in the etiology of numerous non-communicable chronic diseases. During the postprandial state, the levels of 1,2-dicarbonyl compounds can increase, depending on numerous factors, including characteristics of the subjects mainly related to glucose metabolism disorders and nutritional status, as well as properties related to the chemical composition of meals, including macronutrient composition and the presence of dietary bioactive molecules and macromolecules. In this review, we examine the chemical, biochemical, and physiological pathways that contribute to postprandial generation of 1,2-dicarbonyl compounds. The modulation of postprandial 1,2-dicarbonyl compounds is discussed in terms of biochemical pathways regulating the levels of these compounds, as well as the effect of phenolic compounds, dietary fiber, and dietary patterns, such as Mediterranean and Western diets.

Receptor for Advanced Glycation End Products (RAGE): A Pivotal Hub in Immune Diseases

As a critical molecule in the onset and sustainment of inflammatory response, the receptor for advanced glycation end products (RAGE) has a variety of ligands, such as advanced glycation end products (AGEs), S100/calcium granule protein, and high-mobility group protein 1 (HMGB1). Recently, an increasing number studies have shown that RAGE ligand binding can initiate the intracellular signal cascade, affect intracellular signal transduction, stimulate the release of cytokines, and play a vital role in the occurrence and development of immune-related diseases, such as systemic lupus erythematosus, rheumatoid arthritis, and Alzheimer’s disease. In addition, other RAGE signaling pathways can play crucial roles in life activities, such as inflammation, apoptosis, autophagy, and endoplasmic reticulum stress. Therefore, the strategy of targeted intervention in the RAGE signaling pathway may have significant therapeutic potential, attracting increasing attention. In this paper, through the systematic induction and analysis of RAGE-related signaling pathways and their regulatory mechanisms in immune-related diseases, we provide theoretical clues for the follow-up targeted intervention of RAGE-mediated diseases.

Methylglyoxal and high glucose inhibit VEGFR2 phosphorylation at specific tyrosine residues

Diabetes is characterized by hyperglycemia and a significant risk of vascular complications. Vascular endothelial growth factor (VEGF) and its main receptor VEGFR2 (KDR), which is highly expressed in vascular endothelial cells, are essential mediators of vascular maintenance and angiogenesis. During glycolysis after high calorie food intake, methylglyoxal (MGO) is formed and MGO blood levels are elevated in diabetes. MGO reacts with arginine residues to generate MG-H1 or with lysine residues to carboxyethyl lysine which are common components of advanced glycation end-products. Therefore, the question arises whether hyperglycemic conditions affect VEGF signaling via a ligand-independent direct modification of signaling components. As a first step, the effect of MGO on VEGFR2 activation was investigated in cultured endothelial cells from human umbilical vein by determination of VEGFR2 phosphorylation at selected tyrosine residues by ELISA and immunoblotting using phospho-specific antibodies. Phosphorylation of VEGFR2-Y996, VEGFR2-Y1054, or VEGFR2-Y1175 reached a maximum 5 min after stimulation of endothelial cells with VEGF. Phosphorylation was significantly inhibited by 100 µM MGO and to a lesser extent by high glucose treatment. 2,3-Pentanedione and glyoxal were investigated for comparison. In summary, VEGFR2 phosphorylation is sensitive to MGO or high glucose concentrations which may be relevant in the pathophysiology of microvascular disease in diabetes.

AGE/Non-AGE Glycation: An Important Event in Rheumatoid Arthritis Pathophysiology

Rheumatoid arthritis (RA) is a chronic inflammatory, autoimmune disease that gradually affects the synovial membrane and joints. Many intrinsic and/or extrinsic factors are crucial in making RA pathology challenging throughout the disease. Substantial enzymatic or non-enzymatic modification of proteins driving inflammation has gained a lot of interest in recent years. Endogenously modified glycated protein influences disease development linked with AGEs/non-AGEs and is reported as a disease marker. In this review, we summarized current knowledge of the differential abundance of glycated proteins by compiling and analyzing a variety of AGE and non-AGE ligands that bind with RAGE to activate multi-faceted inflammatory and oxidative stress pathways that are pathobiologically associated with RA-fibroblast-like synoviocytes (RA-FLS). It is critical to comprehend the connection between oxidative stress and inflammation generation, mediated by glycated protein, which may bind to the receptor RAGE, activate downstream pathways, and impart immunogenicity in RA. It is worth noting that AGEs and non-AGEs ligands play a variety of functions, and their functionality is likely to be more reliant on pathogenic states and severity that may serve as biomarkers for RA. Screening and monitoring of these differentially glycated proteins, as well as their stability in circulation, in combination with established pre-clinical characteristics, may aid or predict the onset of RA.

Elevated serum levels of methylglyoxal are associated with impaired liver function in patients with liver cirrhosis

Methylglyoxal (MGO) is a highly reactive dicarbonyl species that forms advanced glycation end products (AGEs). The binding of these AGEs to their receptor (RAGE) causes and sustains severe inflammation. Systemic inflammation is postulated to be a major driver in the progression of liver cirrhosis. However, the role of circulating MGO levels in liver cirrhosis remains unknown. In this study, we investigated the serum levels of two dicarbonyl species, MGO and glyoxal (GO) using tandem mass spectrometry (HPLC-MS/MS) and evaluated their association with disease severity. A total of 51 inpatients and outpatients with liver cirrhosis of mixed etiology and different disease stages were included. Elevated MGO levels were seen in an advanced stage of liver cirrhosis (p < 0.001). High MGO levels remained independently associated with impaired liver function, as assessed by the model for end-stage liver disease (MELD) (β = 0.448, p = 0.002) and acute decompensation (AD) (β = 0.345, p = 0.005) scores. Furthermore, MGO was positively correlated with markers of systemic inflammation (IL-6, p = 0.004) and the development of ascites (p = 0.013). In contrast, no changes were seen in GO serum levels. Circulating levels of MGO are elevated in advanced stages of liver cirrhosis and are associated with impaired liver function and liver-related parameters.

Protein oxidation - Formation mechanisms, detection and relevance as biomarkers in human diseases

Generation of reactive oxygen species and related oxidants is an inevitable consequence of life. Proteins are major targets for oxidation reactions, because of their rapid reaction rates with oxidants and their high abundance in cells, extracellular tissues, and body fluids. Additionally, oxidative stress is able to degrade lipids and carbohydrates to highly reactive intermediates, which eventually attack proteins at various functional sites. Consequently, a wide variety of distinct posttranslational protein modifications is formed by protein oxidation, glycoxidation, and lipoxidation. Reversible modifications are relevant in physiological processes and constitute signaling mechanisms ("redox signaling"), while non-reversible modifications may contribute to pathological situations and several diseases. A rising number of publications provide evidence for their involvement in the onset and progression of diseases as well as aging processes. Certain protein oxidation products are chemically stable and formed in large quantity, which makes them promising candidates to become biomarkers of oxidative damage. Moreover, progress in the development of detection and quantification methods facilitates analysis time and effort and contributes to their future applicability in clinical routine. The present review outlines the most important classes and selected examples of oxidative protein modifications, elucidates the chemistry beyond their formation and discusses available methods for detection and analysis. Furthermore, the relevance and potential of protein modifications as biomarkers in the context of disease and aging is summarized.

Sialylated Immunoglobulins for the Treatment of Immuno-Inflammatory Diseases

Immunoglobulins are the potent effector proteins of the humoral immune response. In the course of evolution, immunoglobulins have formed extremely diverse types of molecular structures with antigen-recognizing, antigen-binding, and effector functions embedded in a single molecule. Polysaccharide moiety of immunoglobulins plays the essential role in immunoglobulin functioning. There is growing evidence that the carbohydrate composition of immunoglobulin-linked glycans, and especially their terminal sialic acid residues, provide a key effect on the effector functions of immunoglobulins. Possibly, sialylation of Fc glycan is a common mechanism of IgG anti-inflammatory action in vivo. Thus, the post-translational modification (glycosylation) of immunoglobulins opens up significant possibilities in the diagnosis of both immunological and inflammatory disorders and in their therapies. This review is focused on the analysis of glycosylation of immunoglobulins, which can be a promising addition to improve existing strategies for the diagnosis and treatment of various immuno-inflammatory diseases.

Relationship between serum-soluble receptor for advanced glycation end products (sRAGE) and disease activity in rheumatoid arthritis patients

Objective: Considering the important role of serum soluble receptor for advanced glycation end product (sRAGE/RAGE)-ligand system in rheumatoid arthritis (RA), this study aimed to evaluate serum sRAGE levels in RA patients compared to healthy subjects and to assess whether there is an association between sRAGE levels and disease characteristics in RA.Methods: In this cross-sectional study, 60 RA patients according to the ACR/EULAR 2010 criteria and 30 age- and sex-matched healthy controls were included. In patients, clinical examination was performed and disease activity score 28 (DAS-28) measure of disease activity was assessed. Serum sRAGE level was measured using ELISA kit.Results: The mean ± SD age of patients and controls was 54.86 ± 11.65 and 50.71 ± 3.72 years, respectively). Serum sRAGE level was significantly higher in RA patients (median [25th and 75th percentiles], 1000.3 [792.00, 1486.8]) compared to healthy controls (median [25th and 75th percentiles], 293.25 [220.35, 364.24]) (p < .001). There was significant difference in serum sRAGE level according to the activity of disease (p < .001). There were significant positive correlations between serum sRAGE level with disease activity (r = 0.67, p < .001), ESR (r = 0.411, p = .001) and CRP (r = 0.273, p = .035). There were no significant correlations between serum sRAGE level with demographic characteristics as well as biochemical measurements including serum creatinine, BUN, RF, and Anti-CCP (p > .05).Conclusions: Our study revealed higher serum sRAGE levels in RA patients compared to healthy controls, which correlated positively with disease activity.